Well fluid comprising a fluorinated liquid phase

ABSTRACT

The present invention relates to a well drilling, completion or workover fluid wherein the continuous phase of the fluid essentially consists of a liquid fluorinated compound. In a variant, the liquid fluorinated compound is a perfluorinated compound.

FIELD OF THE INVENTION

The present invention relates to a fluid of optimized compositionintended to solve problems encountered during operations such asdrilling, completion or workover operations in a well.

What is referred to as completion is all the preparation or outfittingoperations required for bringing in a geologic formation from thewellbore. These completion operations use particular fluids calledcompletion fluids.

What is referred to as workover is all the operations carried out in aproducing or potentially producing well. Workover fluids can be used inthe producing well in circulation in a comparable manner to drillingfluids, or in form of a spacer fluid.

A feature that all these drilling, completion or workover fluids have incommon is the physico-chemical suitability of their formulation to thenature of the geologic formations they are in contact with and to themain functions they are intended for. In fact, it is well known that thefluids used in a well generally have most of the following functions:

-   well walls stability maintenance,-   controlled filtration in permeable formations,-   good capacity for cleaning the bottom of the well in circulation,-   easy density adjustment,-   stability in temperature and in time,-   no or little contamination by clays or electrolytes,-   easy manufacture and treatment, etc.

To obtain these complex combined functions, that can be incompatible,the relative amounts of additives suited to the desired functions withinthe context of the well constraints have to be selected and adjusted.Thus, depending on the nature of the geologic formation problemsencountered, of their bottomhole conditions, such as pressure andtemperature, and according to the main functions essential for thefluid, the composition of the well fluid is generally the result of acompromise between several functions and the cost of the essentialadditives.

BACKGROUND OF THE INVENTION

The main problems encountered when using fluids under HP/HT (highpressure/high temperature) conditions are essentially due to thefollowing two constraints:

-   a well fluid density above 2000 kg/m³ is required in order to    balance the bottomhole pressure at great well depths,-   the bottomhole static pressures are often much higher than 200° C.    and they can sometimes reach or even exceed 300° C.

The drilling techniques commonly used under less drastic conditions(less deep reservoir and/or temperature less than or equal to 200° C.)utilize water-base fluids or oil-base fluids. In the latter case, thecontinuous phase generally consists of a hydrocarbon of diesel orisoparaffin type. The density of the fluid is then adjusted by addingdense salts soluble in aqueous phases (barium chloride, alkalineformates, . . . ), and also by adding mineral fillers dispersed in thecontinuous phase. The dispersion stability is generally provided byvarious types of suspending, thinning, wetting agents, polymers.

The fluids currently known are ill-suited to HP/HT conditions, inparticular above 220° C. The main problem of current muds under HP/HTconditions is thus when drilling is stopped. After an extended period oftime in the well bottom, a strongly marked particle settling phenomenon,notably weighting material, appears. In some cases, a solid mass thatcannot be recirculated forms. The well is then plugged.

This phenomenon is linked to the high mineral filler concentration inthe fluid formulation, which is necessary to densify the mud, and to thesettling acceleration due to the loss of the rheological properties ofthe fluid. The loss of Theological properties is notably due to thethermal degradation of the various additives the mud is made up of,notably the suspending agents (such as clays, polymers, surfactants). Asregards hardening of the deposit, gelling of the clays under the effectof temperature is probably involved.

For water-base fluids, hydrosoluble polymers are used as viscosifiersfor the continuous phase. They are of polysaccharide type, such asxanthan derivatives or others, or synthetic polymers, vinyl copolymersor terpolymers, based on acrylamide and comprising at least one of thefollowing monomer units: acrylate, acrylamidomethyl propane sulfonate(AMPS), itaconate, styrene sulfonate. The limit temperature of use forthis type of polymer rarely exceeds 200° C. The following documents canbe mentioned by way of reference: Y. M. Wu, B. Q. Zhang, T. Wu, C. G.Zhang: Colloid Polym. Science 27 (9) 836-84 (2001); A. Audibert, L.Rousseau, J. Kieffer, SPE 50724, (1999); Y. M. Wu, D. J. Sun, B. Q.Zhang, CH. G. Zhang: J. Appl. Polym. Sci., 83, 3068-3075 (2002).

The mineral fillers can also be suspended by adding to the formulationdispersing polymers that are also limited to a similar temperaturerange.

In the case of non aqueous fluids, notably fluids comprisingisoparaffins as the continuous phase, the formulation is more complex.They are reverse water-in-oil emulsions whose continuous phase densityis about 900 kg/m³. When the density of the fluid is necessarily high(density greater than or equal to 2000 kg/m³), the volume fraction offillers incorporated in the fluid can exceed 35%. The stability of theseformulations is linked with the stability of the emulsions at hightemperature and, then again, it is very difficult to formulate stablefluids at temperatures above 220° C. The risk involved is then a phaseseparation that causes degradation of the rheological properties,encroachment of the formations and settling of the weighting agents(P.D. Scott, M. Zamora, C. Aldea, IADC/SPE Drilling Conference, 2-4March, Dallas, Tex.—D. J. Oakley, et al. IADC/SPE Asia Pacific DrillingTechnology, 11-13 September, Kuala Lumpur, Malaysia). The deposits canbe plentiful and gel after some ten hours under the effect oftemperature.

The present invention relates to well fluids comprising dense liquidsstable at high temperatures, which are the main constituents of thecontinuous phase. The invention more particularly relates to well fluidswhose continuous phase comprises a fluorinated dense liquid stable athigh temperature. The preferred range of application of these fluids isgenerally under high pressure and high temperature conditions generallyencountered during drilling or development of very deep-seated geologicreservoirs, i.e. more than 3000 m, generally more than 5000 m in depth.

SUMMARY OF THE INVENTION

The object of the present invention thus consists in using a liquidfluorinated compound as the main constituent of the continuous phase ofwell fluids for at least two essential properties of these products:

-   their high density,-   their temperature resistance.

The fluorinated liquid compounds concerned by the invention can containor not other functional groups, for example: other halogenated groupsother than fluorinated, or hydrocarbon-containing groups. The well fluidaccording to the invention can contain more specifically perfluorinatedliquids as the fluorinated liquid, more specificallyperfluoropolyalkylethers and their derivatives.

The perfluoropolyalkylether family (also referred to asperfluoropolyethers or PFPE) is notably an advantageous variant. PFPEsare a single class of fluorinated polymers that are liquid at ambienttemperature and have particular characteristics such as low interfacialenergy, high lubricating power, very high thermal and chemicalresistance, and low toxicity. Densities from 1800 kg/m³ to 2000 kg/m³and thermal stabilities above 300° C. are obtained. Thus,perfluoropolyalkylethers (CAS No.: 60164-51-4) are commercial productsand they are generally used as lubricants, as heat-carrier, as hydraulicfluid in the aerospace industry, as electrical insulant in electronics.The synthesis of PFPEs is described in the following article: W. C.Bunyard et al., Macromolecules, 32, 8224 (1999).

Examples of PFPE liquid are: Krytox® of the Dupont Company, Fomblin® ofthe Solvay Solexis Company, Demnum® of the Daikin Company.

The liquid compounds concerned by the invention can have the followingcharacteristics:

-   the density of the liquid in question, measured at 20° C., can range    between 1800 and 2200 kg/m³,-   the maximum temperature of use of these liquids can range between    220° C. and 400° C.,-   a kinematic viscosity, measured at 20° C., ranging between 10 and    10000 cSt, more particularly between 10 and 2000 cSt. Their molar    mass can range between 1000 and 30000 g/mol, preferably between 1000    and 10000 g/mol.

The perfluoropolyether compound according to the invention has at leastone of the following structures, or mixtures thereof:E-O—(CF(CF₃)CF₂O)_(m)(CFXO)_(n)-E′  (a)where X is equal to F or CF₃;

-   -   E and E′, equal or different from one another, are CF₃, C₂F₅, or        C₃F₇;    -   m and n are integers such that the m/n ratio ranges between 20        and 1000 and the viscosity of the product is in the 10-4000 cSt        range;        C₃F₇O—(CF(CF₃)CF₂O)_(o)-D  (b)        where D is equal to C₂F₅ or C₃F₇;    -   o is an integer such that the viscosity is in the range of        structure (a);        {C₃F₇O—(CF(CF₃)CF₂O)_(p)—CF(CF₃)—}₂  (c)        where p is an integer such that the viscosity is in the range of        structure (a);        E-O—(CF(CF₃)CF₂O)_(q)(C₂F₄O)_(r)(CFX)_(s-E′)  (d)        where X, E and E′according to structure (a);    -   q, r and s are integers or zero numbers such that the viscosity        is in the range of structure (a);        E-O—(C₂F₄O)_(t)(CF₂O)_(u)-E′  (e)        where E and E′according to structure (a);    -   t and u are integers such that the t/u ratio ranges between 0.1        and 5 and the viscosity is in the range of structure (a);        E-O—(CF₂CF₂CF₂O)_(v)-E′  (f)        where E and E′according to structure (a);    -   v is a number such that the viscosity is in the range of        structure (a);        D-O—(CF₂CF₂O)_(z)-D′  (g)        where D and D′, equal or different from one another, are C₂F₅ or        C₃F₇;    -   z is an integer such that the viscosity is in the range of        structure (a).

According to the invention, a well fluid formulation comprising saidfluorinated liquid compounds is also optimized in order to stabilize thesuspension of the solid fillers with emulsifiers and/or rheologymodifiers. The well fluid is suited to withstand a massive brine inflowfrom the reservoir rock and it is therefore advantageously formulated inform of a reverse water emulsion in a continuous phase comprising thefluorinated liquid. This reverse emulsion can also contain other liquidphases, organic for example, which allows formulation stability to beensured in case of hydrocarbon inflow. The emulsion can contain 5 to 40%by volume of a dispersed brine aqueous phase. The brines can consist ofsodium, potassium or calcium chlorides, sodium or potassium carbonates,alkaline formates. The well fluid can contain a mixture of aqueous fluidand hydrocarbon liquid in emulsion in the fluorinated liquid.

Surfactants are used not only to stabilize the emulsion, but also todisperse the mineral fillers (weighting agent and possibly clays ororganophilic clays) and thus to control the Theological properties ofthe fluid, in particular the thixotropy. The emulsifying system usedallows emulsion stability to be obtained over a wide temperature range.

The surfactants used here are selected so as to be compatible with thecontinuous phase, in particular as regards the fluorinated chemicalnature of this phase. Documents U.S. Pat. No. 5,368,847, U.S. Pat. No.4,990,283 and U.S. Pat. No. 20,040,086,650 describe fluorinatedsurfactants for various formulation types.

The surfactants described hereafter are appropriate to be compatible(i.e. soluble or dispersible) with perfluoropolyether.

The surfactants that are useful for the application affect one or moreof the following properties: surface tension, capacity for wetting solidparticles and notably those of the weighting material used (barite,carbonates or others), and/or HLB of the composition. By usingsurfactant mixtures, it is also possible to stabilize multiphase systemsconsisting of at least one perfluorinated liquid phase and at least oneof the following phases: solid phase, aqueous phase,hydrocarbon-containing oil phase.

The fluorinated surfactants that are the object of the present inventioncan be ionic or non-ionic. The following can be mentioned in particular:

(a) perfluorocarboxylic acids containing 5 to 11 carbon atoms, and theirsalts,

(b) perfluorosulfonic acids containing 5 to 11 carbon atoms, and theirsalts,

(c) perfluorophosphates containing 5 to 11 carbon atoms,

(d) non-ionic surfactants such as the compounds meeting the generalformula Rf(CH₂)(OC₂H₄)_(n)OH, wherein Rf is a partly hydrogenatedperfluorocarbon or fluorocarbon chain, n is an integer at least equal to1,

(e) fluorinated non-ionic surfactants of polyoxyethylenefluoroalkylether type,

(f) mono and dicarboxylic acids deriving from perfluoropolyethers, andtheir salts,

(g) mono and disulfonic acids deriving from perfluoropolyethers, andtheir salts,

(h) perfluoropolyether phosphates, perfluoropolyether diphosphates,particularly efficient for dispersion of the mineral fillers,

(i) perfluorinated cationic or anionic surfactants or those derivingfrom perfluoropolyethers having 1, 2 or 3 hydrophobic side chains,

(j) ethoxylated fluoroalcohols, fluorinated sulfonamides or fluorinatedcarboxamides.

The formulations are constituted according to the final density of thefluid to be used, according to the density of the weighting material,and according to the mass fraction and to the density of the brine.

The two tables hereafter show non limitative formulation examples basedon the production of a well fluid whose density, measured at 20° C., is2200 kg/m³.

For a PFPE, with barite and a saturated CaCl₂ type brine: Table 1 (for 1m³ fluid). Water Fluorinated Volume fractions mass liquid Barite BrineFluorinated fraction mass kg mass kg mass kg liquid Barite Brine 0.001679 520.9 0.0 88% 12%  0% 0.05 1501 589.1 110.0 79% 13%  8% 0.10 1323657.4 220.0 70% 15% 16% 0.15 1144 725.6 330.0 60% 16% 24% 0.20 966 793.8440.0 51% 18% 31% 0.25 788 862.0 550.0 41% 19% 39% 0.30 610 930.2 660.032% 21% 47% 0.35 432 998.4 770.0 23% 22% 55% 0.40 253 1066.7 880.0 13%24% 63%

For a PFPE, with calcium carbonate and a saturated CaCl₂ type brine:Table 2 (for 1 m³ fluid). Fluori- Water nated Calcium Volume fractionsmass liquid carbonate Brine Fluorinated Calcium fraction mass kg mass kgmass kg liquid carbonate Brine 0.00 1347 853.4 0.0 71% 29%  0% 0.05 1125965.2 110.0 59% 33%  8% 0.10 903 1076.9 220.0 48% 37% 16% 0.15 6811188.7 330.0 36% 41% 24% 0.20 460 1300.4 440.0 24% 44% 31% 0.22 3711345.1 484.0 20% 46% 35%

These formulation examples for a dense fluid show that the use of PFPEallows the amount of weighting material to be limited. It is clear that,in conventional cases of water-base or oil-base fluids, the volumefraction of weighting material is necessarily greater to reach the samedensity.

1) A well drilling, completion or workover fluid characterized in thatthe continuous phase of said fluid mainly consists of a liquidfluorinated compound. 2) A fluid as claimed in claim 1, wherein saidcompound is a perfluorinated compound. 3) A fluid as claimed in claim 1wherein said compound comprises at least one perfluorinated group and atleast one halogenated group other than fluorinated, or ahydrocarbon-containing group. 4) A fluid as claimed in claim 1, whereinsaid compound comprises a perfluoropolyether. 5) A fluid as claimed inclaim 1, wherein said compound has a density, determined at 20° C.,ranging between 1800 and 2200 kg m³. 6) A fluid as claimed in claim 1,wherein said compound has a viscosity, determined at 20° C., rangingbetween 10 and 10000 cSt, preferably between 20 and 2000 cSt. 7) A fluidas claimed in claim 1, wherein said compound has a molecular massranging between 1000 and 30000 g/mol, preferably between 1000 and 10000g/mol. 8) A fluid as claimed in claim 4, wherein said perfluoropolyetherhas at least one of the following structures, or mixtures thereof:E-O—(CF(CF₃)CF₂O)_(m)(CFXO)_(n)-E′  (a) where X is equal to F or CF₃; Eand E′, equal or different from one another, are CF₃, C₂F₅, or C₃F₇; mand n are integers such that the m/n ratio ranges between 20 and 1000and the viscosity of the product is in the 10-4000 cSt range;C₃F₇O—(CF(CF₃)CF₂O)_(o-D)  (b) where D is equal to C₂F₅ or C₃F₇; o is aninteger such that the viscosity is in the range of structure (a);{C₃F₇O—(CF(CF₃)CF₂O)_(p)—CF(CF₃)—}₂  (c) where p is an integer such thatthe viscosity is in the range of structure (a);E-O—(CF(CF₃)CF₂O)_(q)(C₂F₄O)_(r)(CFX)_(s)-E′  (d) where X, E andE′according to structure (a); q, r and s are integers or zero numberssuch that the viscosity is in the range of structure (a);E-O—(C₂F₄O)_(t)(CF₂O)_(u)-E′  (e) where E and E′according to structure(a); t and u are integers such that the t/u ratio ranges between 0.1 and5 and the viscosity is in the range of structure (a);E-O—(CF₂CF₂CF₂O)_(v)-E′  (f) where E and E′according to structure (a); vis a number such that the viscosity is in the range of structure (a);D-O—(CF₂CF₂O)_(z)-D′  (g) where D and D′, equal or different from oneanother, are C₂F₅ or C₃F₇; z is an integer such that the viscosity is inthe range of structure (a). 9) A fluid as claimed in claim 1, comprisinga useful amount of at least one surfactant, or mixtures thereof,selected from the following group: (a) perfluorocarboxylic acidscontaining 5 to 11 carbon atoms, and their salts, (b) perfluorosulfonicacids containing 5 to 11 carbon atoms, and their salts, (c)perfluorophosphates containing 5 to 11 carbon atoms, (d) non-ionicsurfactants such as the compounds meeting the general formulaRf(CH₂)(OC₂H₄)_(n)OH, wherein Rf is a partly hydrogenatedperfluorocarbon or fluorocarbon chain, n is an integer at least equal to1, (e) fluorinated non-ionic surfactants of polyoxyethylenefluoroalkylether type, (f) mono and dicarboxylic acids deriving fromperfluoropolyethers, and their salts, (g) mono and disulfonic acidsderiving from perfluoropolyethers, and their salts, (h)perfluoropolyether phosphates, perfluoropolyether diphosphates,particularly efficient for dispersion of the mineral fillers, (i)perfluorinated cationic or anionic surfactants or those deriving fromperfluoropolyethers having 1, 2 or 3 hydrophobic side chains, (j)ethoxylated fluoroalcohols, fluorinated sulfonamides or fluorinatedcarboxamides. 10) A fluid as claimed in claim 1, comprising a determinedamount of inert weighting fillers intended to adjust the densitythereof. 11) A fluid as claimed in claim 10, wherein said inert fillersare dispersed by adding one or more surfactants as claimed in claim 9.12) A fluid as claimed in claim 1, comprising an amount of an aqueousfluid in water emulsion in said liquid compound. 13) A fluid as claimedin claim 12, wherein said aqueous fluid is a brine of sodium, potassiumor calcium chlorides, of sodium or potassium carbonates, or of alkalineformates. 14) A fluid as claimed in claim 12, wherein said emulsion isstabilized by adding one or more surfactants as claimed in claim
 9. 15)A fluid as claimed in claim 1, comprising an amount of ahydrocarbon-containing liquid in water emulsion in said liquid compound16) A fluid as claimed in claim 12, comprising an aqueous fluid and ahydrocarbon-containing liquid in emulsion in said liquid compound. 17)Use of a fluid as claimed in claim 1 for drilling a reservoir requiringa fluid density above 2000 kg m³, at a temperature above 200° C.